Biogas is a combustible gas generated by fermenting organic matters by microorganisms under anaerobic conditions. Such gas is called biogas, since it is firstly discovered in the swamp. Various organic matters, such as human and animal dung, crop straw, sewage and the like, are fermented under anaerobic conditions in a closed methane fermentation device, decomposed and transformed to produce biogas by a wide variety of biogas fermentation microorganisms. Biogas is a mixture of various gases, generally containing 50%-70% of methane, 20%-40% of carbon dioxide, and small amounts of water vapor, nitrogen, hydrogen and hydrogen sulfide etc. as the remainder. The biogas can be decontaminated by removing contaminating gases and by purified by removing carbon dioxide, and the resulting product gas can meet the national standard of natural gas for vehicle.
According to GB18047-2000 for Compressed Natural Gas (CNG) as Vehicle Fuel, the content of carbon dioxide in CNG should be less than or equal to 3.0%. Carbon dioxide is a non-combustible component in biogas, too high content will reduce the heat of combustion of natural gas, and the removal of carbon dioxide from biogas is called biogas purification.
Currently, the practically applied methods for purification of biogas are mainly divided into the following categories:
Physical method: such as pressure swing adsorption method, pressure water washing method, a polymer film separation method and the like;
Chemical method: such as Benfield method, complex catalytic method, steric hindrance amine method, BV potash lye method, ammonia water method and the like;
Physical-chemical method: such as sulfolane-ethanolamine method, MDEA method and the like.
The working principle of the chemical method is to use a chemical agent to absorb the carbon dioxide from biogas, the cost of which is higher due to the need for a large amount of chemical agents.
Three physical methods for biogas purification are mostly applied in the current field of biogas purification.
The working principle of the pressure swing adsorption method is to use the principle of difference in the adsorption property of each component in biogas on the adsorbent and change of adsorption quantity over pressure, to achieve gas separation by cyclical pressure change. In order to ensure continuous gas treatment requirements, the pressure swing adsorption method requires at least two adsorption towers, three towers, four towers or more.
The working principle of the pressure water washing method is to use water to absorb CO2 in raw biogas under pressure. According to the theory of Henry's Law, solubility of gas in the water is directly proportional to the pressure in the same temperature. High-pressure and low temperature are used when water is used to absorb CO2, and low-pressure or high temperature is used when desorbing the gas.
The working principle of the film separation method is that in the presence of driving force formed by the partial pressure of each gases at high-pressure side (feed side) and the low pressure side (permeate side) of polymer film-polyimide hollow fiber filament tube, viz. partial pressure difference, gases with greater solubility coefficient and diffusion coefficient (such as CO2, H2S) preferentially permeate through the tube wall, the remaining gas (CH4) is relatively blocked, so as to achieve the purpose of separation.
The pressure swing adsorption method and the pressure water washing method both have shortcomings such as large space occupation, immovability, and complex process. High-pressure water washing operation has shortcomings such as high cost, high pressure, low temperature, great technical difficulties and easy blockage of absorption tower.
Biogas after desulfurization, cleaning and crude dehydration may be purified by using the pressure swing adsorption method. Specifically, the biogas is compressed first before entering into the refrigeration dryer to dehydrate; and then the compressed biogas enters into the pressure swing adsorption device consisting of two adsorption towers, wherein, impurity gas components such as O2, CO2 are selectively absorbed by the use of special carbon molecular sieve adsorbents loaded in the two adsorption towers, and at last, methane is discharged from the top of the tower.
Biogas after desulfurization, cleaning and crude dehydration may be purified by using pressure water washing method. Specifically, the biogas is compressed in a biogas compressor, and transported into the lower part of the absorption tower, and the CO2 gas is absorbed by heat and mass transfer through countercurrent contact of the biogas with the cooling water spraying down from the upper part of the absorption tower. Part of the outlet gas from the absorption tower returns to a tertiary desorption tower, the other part of the outlet gas is transferred into the molecular sieve adsorbent to remove residual moisture and trace gas CO2 residue in the biogas, so as to obtain qualified product gas.
Biogas after desulfurization, cleaning and crude dehydration may be purified by using the film separation method. Specifically, the biogas is compressed firstly, the compressed gas then enters into refrigeration dryer for dehydration, after purification through a filter to finely remove dust and oil, the purified biogas after heat exchanger enters into the film group for purification.
The maintenance cost for purification of biogas by the pressure swing adsorption method is high because the molecular sieve and valves need to be replaced regularly. The pressure water washing method requires a large amount of recycled water for CO2 absorption, the required control instrumentation valves also need to be replaced regularly, and therefore the maintenance cost is high. A large adsorption tower (absorber) is required to be built for both the pressure swing adsorption method and the pressure water washing method, and the tower has the shortcomings such as large space occupation, immovability and complex process.